The present invention relates to an actuator for a print hammer in an impact-type line printer.
A printing mechanism for a high-speed line printer is composed of plural actuators. Each actuator is composed of a yoke, an electromagnetic coil mounted on the yoke, an armature which is attracted to the yoke each time the electromagnetic coil is excited, and a screw for defining the fixed stationary position of the armature. The armature is rotatably supported by bearings mounted on inner surfaces of respective opposed plates secured to opposite sides of the yoke. The kinetic energy of the armature is transmitted to a print hammer through a pushrod slidably mounted on a guide plate so that the print hammer presses the paper sheet against a printing character member through an ink ribbon to thereby print a character on the sheet. After printing, the print hammer is returned to its initial position by a plunger and a return spring. The actuators are mounted on a mounting base.
In order to achieve a high quality printing with the above construction, it is necessary for the print hammer to move at a speed of 5 to 5.5 m/sec. Generally, the yoke is made of a highly permeable magnetic material having excellent magnetic properties, such as 3% silicon steel. The magnetic material is shaped into the desired configuration by pressing, sintering, a lost wax process or the like. However, since the processability of such types of material is not satisfactory, it is essential that the material be subjected to a machining finishing process to obtain the required dimensional accuracy. Another problem is that the cost of such material is high, which increases the overall manufacturing cost. Further, when the armature is attracted by the electromagnetic coil and is brought into striking engagement with the yoke, the yoke is subjected to an impact force of about 5 to 10 kg. The resulting vibration is transmitted via the mounting base to the adjoining actuator, displacing the armature from its original fixed position. This results in errors such as misalignment of the printed characters.